Bar codes representing product information such as the date of production, names of the manufacturer and distributor, usable term, etc. have heretofore been widely utilized for a variety of kinds of products. Here, the bar codes are such that the encoded information is read out by using a reader. Therefore, a surface on where the bar code is printed must be made flat arousing, however, a problem in that limitation is imposed on the surface where the bar code is printed and on the amount of information that can be encoded especially in the field of packing materials such as bottles and caps.
In recent years, therefore, a technology for displaying information has been utilized by making use of an IC tag. The IC tag is also called an RFID (radio-frequency identification) and is a communication terminal of a very small size of the form of a tag in which an IC chip storing predetermined information is buried together with an r-f antenna in a dielectric material such as a resin or a glass. The IC tag reads the product information stored in the IC chip through radio communication. A memory in the IC chip is capable of storing the data of, for example, several hundreds of bytes, offering an advantage of capability for storing large amounts of product information. Besides, the IC tag can read the stored information in a non-contacting manner without accompanied by such problems as wear due to contact and, further, offers such an advantage in that the IC tag can be worked in a shape to meet the form of the product and in a small size and in a reduced thickness.
As caps provided with the IC tag, patent documents 1 and 2 are proposing those having an IC tag buried in a top plate.    Patent document 1: JP-A-2006-62716    Patent document 2: JP-A-2005-321935